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<h1 class="title toc-ignore">Advanced <code>future</code> and <code>promises</code> usage</h1>



<p>This article discusses the benefits of using <code>promises::future_promise()</code> over a combination of <code>future::future()</code> + <code>promises::promise()</code> to better take advantage of computing resources available to your main R session. To demonstrate these benefits, we’ll walk-through a use-case with the <code>plumber</code> package. (<a href="https://www.rplumber.io/">See here</a> to learn more about <code>plumber</code> and <a href="futures.html">the previous article</a> to learn more about <code>future</code>.)</p>
<div id="the-problem-with-futurepromise" class="section level2">
<h2>The problem with <code>future()</code>+<code>promise()</code></h2>
<p>In an ideal situation, the number of available <code>future</code> <em>workers</em> (<code>future::nbrOfFreeWorkers()</code>) is always <strong>more than</strong> the number of <code>future::future()</code> <em>jobs</em>. However, if a <code>future</code> job is attempted when the number of free workers is <code>0</code>, then <code>future</code> will block the current R session until one becomes available.</p>
<p>For a concrete example, let’s imagine a scenario, where seven <code>plumber</code> requests are received at the same time with only two <code>future</code> workers available. Also, let’s assume the <code>plumber</code> route(s) serving the first 6 requests use <code>future::future()</code> and take ~10s to compute <code>slow_calc()</code>:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1"></a><span class="co">#* @get /slow/&lt;k&gt;</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="cf">function</span>() {</span>
<span id="cb1-3"><a href="#cb1-3"></a>  future<span class="op">::</span><span class="kw">future</span>({</span>
<span id="cb1-4"><a href="#cb1-4"></a>    <span class="kw">slow_calc</span>()</span>
<span id="cb1-5"><a href="#cb1-5"></a>  })</span>
<span id="cb1-6"><a href="#cb1-6"></a>}</span></code></pre></div>
<p>Let’s also assume the <code>plumber</code> route serving the last request does not use any form of <code>future</code> or <code>promises</code> and takes almost no time to compute.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1"></a><span class="co">#* @get /fast/&lt;k&gt;</span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="cf">function</span>() {</span>
<span id="cb2-3"><a href="#cb2-3"></a>  <span class="kw">fast_calc</span>()</span>
<span id="cb2-4"><a href="#cb2-4"></a>}</span></code></pre></div>
<p>The figure below depicts the overall timeline of execution of these 7 requests under the conditions we’ve outlined above. Note that the y-axis is ordered from first request coming in (<code>/slow/1</code>) to the last request (<code>/fast/7</code>).</p>
<p><img src="" width="75%" alt="Early workers take more time than expected. Main R session is blocked" /></p>
<p>Note how R has to wait 20s before processing the 7th request (shown in green). This is a big improvement over not using <code>future</code>+<code>promises</code> at all (in that case, R would have to wait 60s before processing). However, since there are only two <code>future</code> workers available R still has to wait longer than necessary to process that last request because the main R session must wait for a <code>future</code> worker to become available. The video below animates this behavior:</p>
<div class="vembedr" align="center">
<div>
<iframe class="vimeo-embed" src="https://player.vimeo.com/video/505287449" width="533" height="300" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
</div>
</div>
</div>
<div id="the-solution-future_promise" class="section level2">
<h2>The solution: <code>future_promise()</code></h2>
<p>The advantage of using <code>future_promise()</code> over <code>future::future()</code> is that even if there aren’t <code>future</code> workers available, the <code>future</code> is scheduled to be done when workers become available via <code>promises</code>. In other words, <code>future_promise()</code> ensures the main R thread isn’t blocked when a <code>future</code> job is requested and can’t immediately perform the work (i.e., the number of jobs exceeds the number of workers).</p>
<p>Continuing with the example above, we can swap out the calls to <code>future::future()</code> with <code>future_promise()</code>.</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1"></a><span class="co">#* @get /slow/&lt;k&gt;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="cf">function</span>() {</span>
<span id="cb3-3"><a href="#cb3-3"></a>  promises<span class="op">::</span><span class="kw">future_promise</span>({</span>
<span id="cb3-4"><a href="#cb3-4"></a>    <span class="kw">slow_calc</span>()</span>
<span id="cb3-5"><a href="#cb3-5"></a>  })</span>
<span id="cb3-6"><a href="#cb3-6"></a>}</span></code></pre></div>
<p>With this change to <code>future_promise()</code>, note how the <code>/fast/7</code> route now does not have to wait on <code>future</code> work to finish processing. Therefore, <code>plumber</code> can complete the last requests almost immediately:</p>
<p><img src="" width="75%" alt="future_promise() keeps the main R session free" /></p>
<p>The vertical gray bars in the figure above represent timepoints where the main R session is actually busy. Outside of these gray areas, the R session is free to do other things, for example, executing other <code>promises</code> or, more generally, non-<code>future</code> work. The video below animates this behavior:</p>
<div class="vembedr" align="center">
<div>
<iframe class="vimeo-embed" src="https://player.vimeo.com/video/505286442" width="533" height="300" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
</div>
</div>
<div style="font-size: 20px; margin-top: 40px; text-align: right;">
<p>Next: <a href="shiny.html">Using <code>promises</code> with Shiny</a></p>
</div>
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